1. Field of the Invention
The present invention relates to a dental prosthesis suitable for the restoration of the teeth with crowns, bridges, etc.
2. Prior Art
Heretofore, dental prostheses placed in the oral cavity such as crowns or bridges have been required to possess the following properties:
1. Safety in use. PA1 2. Biocompatibility to intra-oral tissue. PA1 3. Corrosion resistance to intr-oral environment (chemical stability). PA1 4. Strength in intra-oral environment (dynamic stability). PA1 5. Physical durability in intra-oral environment. PA1 6. Fitness to abutment teeth. PA1 7. Esthetics. PA1 (A) a dental porcelain material, and PA1 (B) one or (two or) more metals selected from the group consisting of Ti, Zr, Au, Pt, Pd, Ag, In and Sn, and said outer layer is a dental porcelain material.
Of these properties, weight has recently been placed on esthetics. For that reason, frequent use has been made of dental prostheses made of the so-called metal baked porcelain including an inner layer portion prepared with an alloy to be baked with porcelain by the lost wax casting technique and an outer layer portion formed of dental porcelain baked to the inner layer portion or dental prostheses made of the so-called all-ceramics including an inner layer portion prepared by building dental porcelain up on a metal foil or refractory material and baking it, and an inner layer portion to which the outer layer portion of the dental porcelain is baked. Comparatively lately, further dental prostheses have been developed, including an inner layer portion made of an electric conductive sintered ceramic product by the ram-type electric discharge machining and an outer layer portion to which dental porcelain is baked.
However, the dental prostheses made of the metal baked porcelain are disadvantageous in that they may give rise to a discoloration of the gingival margin in the oral cavity or become poor in corrosion resistance when a base metal alloy is used for the inner layer portions, and pose a problem of safety, e.g., an allergic problem due to alloy components. Another problem is that, prepared by the lost wax casting process, the inner layer portions often fail to possess sufficient strength due to casting defects such as porosities, or become so poor in the accuracy of fitness to abutment teeth, as occurring with a bridge applied across a number of teeth, that no sufficient accuracy of fitness to abutment teeth can be obtained even by relying upon soldering. The application of a noble metal alloy to the inner layer portions has the disadvantage of causing a thermal deformation in thin regions such as marginal regions.
On the other hand, the dental prostheses made of ceramics in their entirety have an inner layer portion prepared by the building-up and baking of dental porcelains. However, they have the disadvantages that they are inferior in the accuracy of fitness to abutment teeth because the shrinkage of dental porcelain is increased upon baking. They can neither be applied in the form of such specific dental prostheses as bridges because general dental porcelain has reduced or limited strength, although alumina porcelain is of higher strength.
Recently developed dental prostheses including inner layer portions formed of an electric conductive sintered ceramic product of titanium boride have the advantages that the inner layer portions excel in the accuracy of fitness to abutment teeth because of being formed by the ram-type electric discharge machining, are so much higher in strength than dental porcelain, as expressed in terms of a bending strength of about 700 MPa and a fracture toughness value of about 4.0 MN/m.sup.3 2, that they can be used for an inner layer portion of a bridge applied across a number of teeth, and show reduced or limited deformation upon the baking of porcelain. However, they have the disadvantages that no sufficient bonding strength is obtained because they can hardly be diffused between the electric conductive sintered ceramic product and porcelain baked as an outer layer portion, and their coefficient of thermal expansion is as low as 4 to 6.times.10.sup.-6 /.degree. C. so that it is impossible to use as an outer layer portion porcelain to be baked with a metal, which has a coefficient of thermal expansion of, e.g., 10 to 20.times.10.sup.-6 /.degree. C., thus making it necessary to select and use a dental porcelain material having a low coefficient of thermal expansion. In some cases, the dental porcelain baked as the outer layer portion may crack due to a difference in the coefficient of thermal expansion. Additionally, the electric conductive sintered ceramic product of titanium boride involves difficulty in its shape modification because of their hardness Hv being as high as 3350.